package com.primaryedu.encrpy;

import java.io.BufferedInputStream;
import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.FileInputStream;
import java.io.FilterInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.UnsupportedEncodingException;

public class Base64Decoder extends FilterInputStream {  
	  
	private static final char[] chars = {  
	 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J',  
	 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T',  
	 'U', 'V', 'W', 'X', 'Y', 'Z', 'a', 'b', 'c', 'd',  
	 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n',  
	 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x',  
	 'y', 'z', '0', '1', '2', '3', '4', '5', '6', '7',  
	 '8', '9', '+', '/'  
	};  
	  
	// A mapping between char values and six-bit integers  
	private static final int[] ints = new int[128];  
	static {  
	 for (int i = 0; i < 64; i++) {  
	   ints[chars[i]] = i;  
	 }  
	}  
	  
	private int charCount;  
	private int carryOver;  
	  
	/*** 
	* Constructs a new Base64 decoder that reads input from the given 
	* InputStream. 
	* 
	* @param in the input stream 
	*/  
	public Base64Decoder(InputStream in) {  
	 super(in);  
	}  
	  
	/*** 
	* Returns the next decoded character from the stream, or -1 if 
	* end of stream was reached. 
	* 
	* @return  the decoded character, or -1 if the end of the 
	*      input stream is reached 
	* @exception IOException if an I/O error occurs 
	*/  
	public int read() throws IOException {  
	 // Read the next non-whitespace character  
	 int x;  
	 do {  
	   x = in.read();  
	   if (x == -1) {  
	     return -1;  
	   }  
	 } while (Character.isWhitespace((char)x));  
	 charCount++;  
	  
	 // The '=' sign is just padding  
	 if (x == '=') {  
	   return -1;  // effective end of stream  
	 }  
	  
	 // Convert from raw form to 6-bit form  
	 x = ints[x];  
	  
	 // Calculate which character we're decoding now  
	 int mode = (charCount - 1) % 4;  
	  
	 // First char save all six bits, go for another  
	 if (mode == 0) {  
	   carryOver = x & 63;  
	   return read();  
	 }  
	 // Second char use previous six bits and first two new bits,  
	 // save last four bits  
	 else if (mode == 1) {  
	   int decoded = ((carryOver << 2) + (x >> 4)) & 255;  
	   carryOver = x & 15;  
	   return decoded;  
	 }  
	 // Third char use previous four bits and first four new bits,  
	 // save last two bits  
	 else if (mode == 2) {  
	   int decoded = ((carryOver << 4) + (x >> 2)) & 255;  
	   carryOver = x & 3;  
	   return decoded;  
	 }  
	 // Fourth char use previous two bits and all six new bits  
	 else if (mode == 3) {  
	   int decoded = ((carryOver << 6) + x) & 255;  
	   return decoded;  
	 }  
	 return -1;  // can't actually reach this line  
	}  
	  
	/*** 
	* Reads decoded data into an array of bytes and returns the actual  
	* number of bytes read, or -1 if end of stream was reached. 
	* 
	* @param buf the buffer into which the data is read 
	* @param off the start offset of the data 
	* @param len the maximum number of bytes to read 
	* @return  the actual number of bytes read, or -1 if the end of the 
	*      input stream is reached 
	* @exception IOException if an I/O error occurs 
	*/  
	public int read(byte[] buf, int off, int len) throws IOException {  
	 if (buf.length < (len + off - 1)) {  
	   throw new IOException("The input buffer is too small: " + len +   
	" bytes requested starting at offset " + off + " while the buffer " +  
	" is only " + buf.length + " bytes long.");  
	 }  
	  
	 // This could of course be optimized  
	 int i;  
	 for (i = 0; i < len; i++) {  
	   int x = read();  
	   if (x == -1 && i == 0) {  // an immediate -1 returns -1  
	     return -1;  
	   }  
	   else if (x == -1) {       // a later -1 returns the chars read so far  
	     break;  
	   }  
	   buf[off + i] = (byte) x;  
	 }  
	 return i;  
	}  
	  
	/*** 
	* Returns the decoded form of the given encoded string, as a String. 
	* Note that not all binary data can be represented as a String, so this 
	* method should only be used for encoded String data.  Use decodeToBytes() 
	* otherwise. 
	* 
	* @param encoded the string to decode 
	* @return the decoded form of the encoded string 
	*/  
	public static String decode(String encoded) {  
	 return new String(decodeToBytes(encoded));  
	}  
	  
	/*** 
	* Returns the decoded form of the given encoded string, as bytes. 
	* 
	* @param encoded the string to decode 
	* @return the decoded form of the encoded string 
	*/  
	public static byte[] decodeToBytes(String encoded) {  
	 byte[] bytes = null;  
	 try {  
	   bytes = encoded.getBytes("UTF-8");  
	 }  
	 catch (UnsupportedEncodingException ignored) { }  
	  
	 Base64Decoder in = new Base64Decoder(  
	                    new ByteArrayInputStream(bytes));  
	   
	 ByteArrayOutputStream out =   
	   new ByteArrayOutputStream((int) (bytes.length * 0.67));  
	  
	 try {  
	   byte[] buf = new byte[4 * 1024];  // 4K buffer  
	   int bytesRead;  
	   while ((bytesRead = in.read(buf)) != -1) {  
	     out.write(buf, 0, bytesRead);  
	   }  
	   out.close();  
	  
	   return out.toByteArray();  
	 }  
	 catch (IOException ignored) { return null; }  
	}  
	  
	public static void main(String[] args) throws Exception {  
	 if (args.length != 1) {  
	   System.err.println("Usage: java Base64Decoder fileToDecode");  
	   return;  
	 }  
	  
	 Base64Decoder decoder = null;  
	 try {  
	   decoder = new Base64Decoder(  
	             new BufferedInputStream(  
	             new FileInputStream(args[0])));  
	   byte[] buf = new byte[4 * 1024];  // 4K buffer  
	   int bytesRead;  
	   while ((bytesRead = decoder.read(buf)) != -1) {  
	     System.out.write(buf, 0, bytesRead);  
	   }  
	 }  
	 finally {  
	   if (decoder != null) decoder.close();  
	 }  
	}  
}  